1. Field of the Invention
The invention relates to the field of pneumatic gauging devices.
2. Prior Art
Pneumatic gauging devices for measuring dimensions are well-known in the art. These devices are used for precision inspection of both inside and outside dimensions. They are most popularly used, however, to inspect internal dimensions such as bores. Most often these devices include a probe having a plurality of fixed orifices (hereinafter a "bore" shall be used for purposes of explanation). The probe is inserted into a workpiece having an unknown internal dimension. The air flow through or back pressure created by the probe is noted. This flow is compared to the flow or pressure created by a bore of known dimension to provide an indication of the deviation of the unknown bore from the known bore. Most often two or more known bores (hereinafter referred to as "standards") are first used to calibrate the apparatus. Prior art pneumatic gauging devices are shown in U.S. Pat. Nos. 2,749,742 and 2,755,659.
It is difficult to calibrate prior art air gauging devices. In fact, it takes a relatively skilled technician to calibrate such devices; once calibrated a lower degree of skill is required to use the devices. The primary reason for the difficulty in calibrating such devices is that pneumatic adjustments are used for calibration. Typically, a number of flow or pressure control adjustments (such as with valves) are used to calibrate the devices. Adjusting one valve frequently requires the readjustment of another valve. Thus, these devices require adjusting and readjusting the same valves, in a trial and error manner, until calibration is achieved.
With the present invention the pneumatic circuit remains fixed and calibration is accomplished in a straight-forward manner with electrical adjustments. This greatly simplifies the calibration procedure and provides a more stable instrument. It allows optimization of the pneumatic components. Also, it permits a wide variation of probe designs to be calibrated for measurement. In the prior art, only probes designed especially for a given instrument could be used.
For the most part, fluid driven gauges are used in the prior art gauging apparatuses. In one instance an electrically driven meter movement is employed for a display. The meter is driven from a differential pressure transformer. However, pneumatic adjustments are still employed for calibration.